The study of acute respiratory infections in the community of Tecumseh, Michigan was carried out in two periods, 1966-71 and 1976-81. During the first period, 267 rhinoviruses were isolated. While the number of total serotypes represented was large, a limited number of types was responsible for more than half of rhinovirus-associated illness, indicating differences in infectivity. Additionally, the number of serotypes that could not be typed increased sequentially, suggesting a change in prevalance of types over time. In the 1976-81 period, 178 rhinoviruses were isolated but it has not yet been possible to type them. It is proposed to raise antisera in guinea pigs systematically against the untypable strains of the 1966-71 period to define their number. The strains isolated in 1976-81 will be typed by microneutralization using antisera already in hand. This work will help in determining if serotypes are continually evolving or if recycling is occurring. ELISA tests will be used to evaluate the question of shared antigens among strains, and statistical techniques will be used to group them in serologic clusters. Finally, human sera collected in Tecumseh will be tested to evaluate the questions of possible recycling of strains, their differential infectivity, and the importance of antibodies to the shared antigens in protection. These results will help in determing the feasibility of immunoprophylaxis of rhinovirus infections.